Electronic circuits operate as the result of the interconnection and interaction of numerous electronic components. Typically, the various electronic components are interconnected using a circuit board.
Rigid printed circuit board technology is very well known. On a substrate, conductive traces are formed based upon a predetermined pattern. At certain locations on the circuit board, conductive pads are formed that couple to the conductive traces. The conductive pads define locations on the circuit board to which electronic components are electrically coupled, for example, by soldering. With all of the required electronic components secured to the circuit board, the conductive traces interconnect the electronic components so that the circuit board becomes operable to perform one or many functions for which it was designed.
A material that is well known in the construction of circuit boards is a type of epoxy glass known as FR4. A typical circuit board will have numerous laminated layers of epoxy glass to provide a relatively stiff, and structurally stable substrate onto which the conductive traces and pads may be formed and to which the electronic components may be secured.
Some circuit applications require the circuit board to be flexible. In some flexible circuit applications, a portion of the circuit board may flex during normal operation of the device to which it is associated. For example, in some applications mechanical components to which the circuits are fixed are required to move relative to each other during normal operation. If numerous interconnections are required between the circuit elements coupled to the respective mechanical components a wired interconnection may not be practical. Advantageously, a flexible circuit board may be used with the interconnections being formed as conductive traces on the flexible circuit board.
The FR4 material has not been suitable for use in flexible circuits. As described above, the FR4 material is typically a construction of numerous (for example, between about 2-5 layers) layers of glass weaves within an epoxy resin. Individually and/or collectively, once the epoxy resin is cured, the layers are stiff. A substitute material that has been successfully used to form flexible circuits is polyimide. Polyimide, however, is substantially more expensive than FR4.
A particularly useful application for flexible circuit boards is in three-dimensional (3D) packaging. In a 3D packaging application, a planar circuit board is laid out and the circuit elements are secured to the circuit board in this planar form. The planar board is then folded one or more times to facilitate packaging of the circuit within a housing and to reduce the volume occupied by the circuit package. Constructing the circuit in a planar form simplifies and facilitates manufacturing using automation. The ability to then fold the circuit board into a more compact volume is a great advantage to the packaging designer, and particularly to the automotive electronic packaging designer, who is frequently faced with space constraints. However, such folding does require at least a portion of the circuit board where it is to be folded to be flexible.
Unlike applications where the circuit board must flex repeatedly during normal operation, flexible circuit boards in 3D packaging applications may be considered "static flexible." That is, the circuit board needs to flex only once when manufactured and folded into its final configuration. Although the circuit board is only required to flex once, it may require forming a bend in the circuit board of relatively small radius, e.g., less than 120 mils. These bending requirements again limit construction of the circuit boards to very flexible materials, i.e., polyimide, because typical FR4 materials are again too brittle to form bends of such small radius. Forming bends in FR4 material of radius less than about 200 mils typically results in cracking of the circuit board, which is an unacceptable defect.
It would be desirable and advantageous to form a flexible circuit board for a 3D packaging application using less expensive materials, such as FR4, without the associated limitations on the ability to form bends in the circuit board.